1. Field of the Invention
The present invention relates to a communication device performing data transmission using a balanced transmission line, and a related art thereof.
2. Description of the Related Art
Balanced transmission systems are known for transmitting data in a differential mode to a balanced transmission line that comprises a pair of communication cables. As one of such balanced transmission systems, there is a power line communication system in which, superimposing a high frequency signal, data is transmitted by a pair of metallic cables that are originally used for power transmission.
The power line communication system is expected to spread to a general house with rapid development of information appliances. The power line communication system in a general house possesses such an advantage that a power line already installed in the house can be used as a balanced transmission line without laying a new cable, when a plurality of computers are to be mutually connected or when a computer is to be connected to other information appliances in the house.
When data transmission is performed using a pair of such metallic cables, there is a problem of EMC (Electromagnetic Compatibility) caused by the unbalance of the pair of the metallic cables used as the transmission line.
FIG. 8 is an illustration showing a balanced transmission system according to the prior art. In FIG. 8, a transmitter 1 and a receiver 2 perform data transmission via a balanced transmission line consisting of metallic cables (a line 3 and a line 4). In the balanced transmission system, the balanced transmission line is originally required to be completely balanced all the way from the transmitter 1 to the receiver 2. However, in fact, there are unbalance factors in circuits of the transmitter 1 and in circuits of the receiver 2, and unbalance components somewhere along the line. Therefore, it cannot be necessarily said that the balanced transmission line is completely balanced.
For example, in the power line communication system using the power line in the house, suppose that the transmitter 1 of FIG. 8 is a modem (for a personal computer) that is connected to an outlet of a living room, and that the receiver 2 is a modem (for a printer) that is connected to an outlet of a study room. When the personal computer in the living room and the printer in the study room perform balanced transmission of data via the power line in the house, another home appliance such as a cleaner is inserted into another outlet in the same house, then the cleaner works as a leakage resistance 6 of FIG. 8, and becomes one of the unbalanced factors for the balanced transmission line (the line 3 and the line 4).
Because of such imperfect balance of the balanced transmission line constituting the balanced transmission system, the EMC problem as mentioned above occurs.
In the EMC problem, there is influence that an unwanted radio wave radiation from the balanced transmission line (the line 3 and the line 4) of the FIG. 8 hurts the other appliances, and influence that a noise entering to the balanced transmission line (the line 3 and the line 4) from the other appliances hurts the balanced transmission system concerned. When the degree of balance of the balanced transmission line deteriorates, the unwanted radio wave radiation increases and a greater problem is caused. The above-mentioned EMC problem is referred to in detail in a reference 1 (“The electromagnetism noise problem of a telecommunications system and its countermeasure technique,” written by Ken Ideguchi and the other, published by Morikita Shuppan on Nov. 25, 1997, P. 99-134).
Therefore, in order to solve the EMC problem in the balanced transmission system, an precise measurement of the unwanted radio wave radiation resulting from deterioration in the degree of balance of the balanced transmission line and a reduction measure for the unwanted radio wave radiation based on the result of the measurement are important.
However, the prior balanced transmission system cannot fully solve the problem mentioned above.